i agree completely with all of MrMonroe's points. those are all exactly the reasons why they do it.
but even when i was still mentally in, it was a bit of a stumbling point to me that the preaching was only done door to door. i figured that in the apostle's day, without today's technologies, perhaps going door to door was the best way to reach all the people, but i always wondered if our message is so important, urgent, and life-saving, then why aren't we making sure it's spread everywhere on billboards, radio, tv, all forms of internet (not just an official WT site, but on forums, youtube, twitter, fb, etc). the official WT line to answer that is that jesus instructed us to go door to door in pairs, and that it's also still the most effective way to reach people.. well, i never quite agreed with that, since a two-hour saturday morning outing usually boils down to some time at coffee shops, driving around, bathroom breaks, some not at homes and maybe talking to about 3 people, 2 of which say "not interested", so that doesn't seem very "effective" to me; and also i figured jesus' point wasn't so much "go door to door" as it was "get the message out there".
..but hey, if the WT says so!